The invention pertains to avionics test equipment, and more particularly to test circuitry for simulating the feedback functions of electrohydraulic control subsystems that move the controllable airfoils, such as ailerons, elevators, spoilers, et cetera, in response to automatic piloting signals.
In airfoil control subsystems of the type pertinent to the present invention, control surfaces are moved by hydraulic actuators that are in turn responsive to electrical control signals such as those generated by an autopilot. The actuator module is sometimes called a power control actuator (PCA) and this actuator may in turn encompass one or more smaller, piloting valves (sometimes called a T-valve as an abbreviation for "transfer" valve) that respond to electrical control signals and pilot a larger actuator valve that performs the work of moving the airplane surface. To drive power control actuators to desired command positions, feedback signals are produced by transducers responsive to the physical movement of the aircraft control surfaces and fed back to the source of the command signals, usually an autopilot. The feedback-generating transducer that performs this function is, in many cases, packaged with the PCA and physically coupled to the mechanical output of the actuator, in this way moving with the actuator output and, hence, in concert with the movement of the control surface. One common type of position transducer is a device called a linear variable differential transformer (LVDT), which comprises a transformer in which the primary and secondary windings are variably coupled by a movable ferromagnetic slug that is mechanically linked to the output of the PCA. An AC reference signal of known amplitude and phase is applied to the primary winding of the LVDT and the output AC signal produced at the LVDT secondary winding varies in amplitude and phase according to the physical movement of the PCA and the associated aircraft control surface.
The electrical command signals that are applied to the PCA are generated by electronic subsystems that govern the flight or assist the pilot in manual flight, and these electronic subsystems may be collectively referred to as automatic pilot controls or autopilots. To test these automatic pilot controls, two alternative techniques have previously been used. First, the automatic pilot controls may be checked out by assembling the entire aircraft system including the PCAs and associated LVDTs and then operating the assembled system while observing movement of the control surfaces and simultaneously taking measurements of the command and feedback signals associated with the PCAs. Another test procedure involves taking measurements of the command signals produced by the automatic pilot control, and then manually adjusting the physical states of dummy LVDTs to produce representative AC feedback signals for application to the automatic pilot control to close the feedback loop.
It will be apparent that both of these testing schemes have severe disadvantages. The first scheme which requires the assembly and connection of all of the aircraft components, including the PCAs and control surfaces, is undesirable for production testing of the automatic pilot controls which need to be checked out before they are installed in the completed aircraft system and, if at all possible, the check-out should be done under laboratory conditions. Additionally, there are certain testing circumstances in which the PCAs and the associated control surfaces are available for complete system testing, but visual verification of the movement of the control surfaces is difficult or impossible, making this first testing procedure inappropriate. The second of the above test procedures is time-consuming, unreliable, and fails to subject the automatic pilot to dynamic loop responses representative of actual feedback control conditions. The tedious process of manually generating feedback signals for the automatic pilot greatly reduces the productivity and accuracy of the testing process.
One previous and unsuccessful effort to overcome such disadvantages was to employ a set of dummy LVDTs, in which the position-sensing ferromagnetic slugs in the LVDTs were controllably moved by drive motors, which in turn were responsive to electrical command signals. However, this attempted simulation failed to operate satisfactorily because of mechanical malfunctions, and alignment problems.